Molecular Packing and Its Effects on Light-emitting Properties of Poly(1,4-phenylenevinylene)s

by Huang, Yi-Fang

Abstract (Summary)

ABSTRACT
Structural evolution and its effect on optical absorption/emission behavior of derivative of PPVs upon isothermal heat treatment at elevated temperatures were studied by means of a combination of polarized light microscopy, x-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, and photoluminescence spectroscopy.
The main physical picture drawn from results of this study over a series of PPVs with flexible side-chains may be summarized as the following:
(1) They are generally liquid-crystalline in nature, typically biaxially nematic in optical texture but morphologically characterized as of lamellar or hexagonal columnar structure. This is consistent with the nematogenic nature one would expect from the rigid backbone as well as the smectogenic nature one would expect from the aliphatic side-chains.
(2) The aggregates formed in solutions and the supramolecular assemblies formed in the bulk state are structurally similar (in terms of the similar level of conjugation), and hence possibly of the same thermodynamic origin. This surfactant-like self-ordering behavior is consistent with the tendency towards segregation between the aromatic, rigid backbone and the aliphatic, flexible side-chains.
(3) The collapse of these conjugated polymers with flexible side-chains into aggregates appears to be a general phenomenon upon slow to moderate solvent removal and not limited to the present case of poor solvency power. This is consistent with the strong tendency toward phase separation in rigid rod solutions delineated by Flory some 30 years ago.
(4) All the above observations may be explained in terms of lyotropic or thermotropic self-assembly of hairy-rod chains into coiled helical conformation with ellipsoidal cross section for the conjugated backbone as shown schematically in Figure 4-42 and 4-43. The flexible side-chains generally tend to fill the space within the ellipsoidal cylindrical structure. As the side-chain length is increased, the increased Van der Waals attraction among side-chains results in more extended period of helical twist or more straighten backbone conformation, rendering preference of lamellar structure over hexagonal helical structure.
(5) As a consequence, supramolecular aggregation is basically enhanced by increased side-chain length or backbone rigidity. This in turn results in more extended conjugation length or more fully developed